1. Field of the Invention
The present invention generally relates to ultrasonic measurement techniques for non-destructive evaluation of materials and structural members, including the determination of fatigue damage.
2. Background
Non-Destructive Evaluation (NDE) of materials involves the inspection of materials without having to damage the materials or dismantle structures to which the materials are incorporated. Among the many important NDE operations are the inspection of aircraft, bridge and building structural members to detect fatigue damage that could possibly lead to catastrophic failure.
Known methods of fatigue damage detection include bombarding material under test with acoustic finite amplitude waves and examining the response waves (including the fundamental and harmonics) produced by the material to determine a nonlinearity parameter (xcex2) which may be correlated to material fatigue. See, pending U.S. patent application Ser. No. 09/065,986, filed Apr. 24, 1998, entitled xe2x80x9cMethod and Apparatus to Assess Optimum Strength During Processing of Precipitation Strengthened Alloysxe2x80x9d, which is hereby incorporated herein by reference as if set forth in its entirety. Among the critical shortcomings of known ultrasonic fatigue damage test methods (such as dye penetrant techniques) and systems is the inability of those approaches to detect material damage such as internal cracks and other important internal eccentricities.
Other restrictions of known acoustic nonlinearity parameter measurement techniques make such techniques less than ideal for field testing. For example, xe2x80x9cdirect substitutionalxe2x80x9d techniques require extensive electronic calibration of the measuring equipment prior to use in order to accurately measure response wave amplitudes necessary for calculating the xcex2.
In view of the aforementioned problems and deficiencies of the known art, the present invention provides a method of measuring acoustic nonlinearity in materials. The method at least includes the steps of a) generating and applying an acoustic signal to a reference material having a known acoustic non-linearity parameter (xcex2), and b) applying an output signal from the reference material and derived from the acoustic signal generated in step a) to at least one environmentally controlled bandpass amplifier. The method also at least includes the steps of c) generating and applying an acoustic signal to a sample material, d) applying an output signal from the sample material and derived from the acoustic signal generated in step c) to at least one environmentally controlled bandpass amplifier, e) comparing the outputs of the bandpass amplifiers in steps b) and d), and f) based upon the comparison in step e), determining the xcex2 of the sample material.
The present invention also provides a system adapted to measure nonlinearity in materials, at least including an acoustic signal generator adapted to generate and apply acoustic signals to a reference material having a known acoustic non-linearity parameter (xcex2) and a sample material with an unknown xcex2, and an acoustic signal detector adapted to be coupled to the reference material and to the sample material, and adapted to apply an output signal from the reference and sample materials derived from the acoustic signals generated by the acoustic signal generator to at least one environmentally controlled bandpass amplifier. The system also at least includes a comparator adapted to compare the outputs of the bandpass amplifier derived from the reference material and the sample material, and an acoustic non-linear parameter calculator coupled to the output of the comparator adapted to determine the xcex2 of the sample material.